Technical Field
The present disclosure relates to a control device for a power converter, in particular a converter for power factor correction (PFC) and further relates to a corresponding control method.
Description of the Related Art
Switched-mode power supplies are known, which are designed to convert a quantity received at an input, for example an AC voltage from the electrical mains supply, into a regulated output quantity, for example a DC voltage, for supplying an electrical load, for instance a group of LEDs.
These power supplies must generally satisfy stringent requirements as regards the corresponding electrical performance; for example, they must guarantee a high quality factor and a substantially unitary power factor.
For this reason, it is generally envisaged to use, in an input stage of the power supply, a power converter of the so-called power-factor-correction (PFC) type, controlled by a purposely provided control device for regulating the power factor during absorption from the electrical mains supply.
By way of example, FIG. 1 shows the circuit diagram of a PFC converter of a boost type, designated as a whole by 1, controlled by a corresponding control device, designated by 2 (it is emphasized, however, that what follows may be applied to different types of converters, for example of a flyback or buck-boost type).
The control device 2 is provided as an integrated circuit, and has a package and corresponding input and output pins; the integrated circuit may be mounted on a same printed-circuit board (PCB) with the circuit components of the PFC converter 1.
In particular, the PFC converter 1 has, in this configuration: an input terminal IN, present on which is a DC input voltage Vin, that is generated by a rectifier stage (not illustrated herein) starting from an AC supply voltage VAC, for example supplied by the electrical mains supply; and an output terminal OUT, connected to which is a charge-storage element 4, in particular a capacitor, present on which is an output voltage Vout, for example a DC voltage, having a value greater than the input voltage Vin and regulated at a desired value (for example, 400 V).
The PFC converter 1 comprises: an inductor 5, connected between the input terminal IN and a first internal node N1; a switch element 6, in particular a MOSFET, connected between the first internal node N1 and a second internal node N2; a sensing resistor 7, connected between the second internal node N2 and a ground reference terminal (GND); and a diode element 8, having its anode connected to the first internal node N1 and its cathode connected to the output terminal OUT.
The switch element 6 has a first current-conduction terminal, in particular the drain terminal of the respective MOSFET, connected to the first internal node N1, a second current-conduction terminal, in particular the source terminal of the respective MOSFET, connected to the second internal node N2, and a control terminal that coincides with the gate terminal of the respective MOSFET.
On the second internal node N2 a control voltage VCS is further acquired, which is a function of the current that flows in the inductor 5, in given operating conditions.
The PFC converter 1 further comprises an auxiliary winding 9, which is magnetically coupled to the inductor 5 and on which a control voltage VZCD is present.
The control device 2 has: an input pin 2a, which is designed to receive a control voltage Vc_in indicative of the input voltage Vin, from a resistive divider 10a, connected to the input terminal IN and formed by a first voltage-dividing resistor and by a second voltage-dividing resistor, which define between them a feedback node on which the control voltage Vc_in is present; an input pin 2b, which is designed to receive a second control voltage Vc out indicative of the output voltage Vout, from a resistive divider 10b, which is connected to the output terminal OUT and is formed by a respective first voltage-dividing resistor and second voltage-dividing resistor, defining between them a respective feedback node on which the second control voltage Vc_out is present; an input pin 2c, which is designed to be connected to the auxiliary winding 9 and to receive the control voltage VZCD, which is a function of the voltage across the auxiliary winding 9; an input pin 2d, which is designed to be connected to the second internal node N2 and to receive the control voltage VCS, which is a function of the voltage across the sensing resistor 7; and an output pin 2e, which is designed to be connected to the control terminal of the switch element 6 and to supply a driving voltage VGD for controlling switching of the switch element 6 in pulse-width modulation (PWM).
In particular, the control device 2 may be configured to control operation of the PFC converter 1 in the so-called transition mode (which is also defined as “critical conduction” or “boundary conduction” mode).
At each switching cycle, the control device 2 controls closing of the switch element 6 during an ON interval Ton (ON interval of the duty cycle), during which the current coming from the supply circulates in the inductor 5 and in the switch element 6 towards ground, determining storage of energy in the same inductor 5.
The duration of the ON interval Ton is determined by the control device 2 through a purposely provided feedback-control loop based on the value of the output voltage Vout, in particular on the value of the control voltage Vc_out indicative of the output voltage Vout, which is compared to an appropriate reference voltage. In a way not described in detail, the control loop may also be based upon a peak-current control.
Next, the control device 2 controls opening of the switch element 6 during an OFF interval Toff (OFF interval of the duty cycle), during which the energy previously stored in the inductor 5 is transferred to the load and to the charge-storage element 4.
In particular, upon completion of the energy transfer, the current in the inductor 5 is zero. The voltage at the first internal node N1, designated hereinafter as “phase voltage Vph”, comes to satisfy a resonance condition around the value of the input voltage Vin on account of the capacitance present on the first internal node N1, mainly due to the parasitic capacitance on the drain terminal of the MOSFET of the switch element 6 and to the parasitic capacitance of the diode element 8 (being in an off condition).
This resonance phase terminates (once again giving rise to the energy-storage phase) when the voltage on the first internal node N1 reaches a lower threshold equal to 2·Vin−Vout, or equal to 0 in the case where this expression yields a value lower than 0.
If the switch element 6 is closed (and the corresponding MOSFET turned on), at this instant, i.e., at the minimum of the resonant oscillation present on the drain voltage of the corresponding MOSFET, when energy transfer is completed, the converter operates in a switching condition at zero current and voltage, enabling a high efficiency. This control is defined as “zero-current detection” (ZCD).
Zero-current detection, and thus determination of the duration of the OFF interval Toff, are carried out by the control device 2 on the basis of the control voltage VZCD, which is in turn a function of the voltage across the auxiliary winding 9. The control voltage VZCD thus is indicative of the zero-current (and zero-voltage) condition in the inductor 5.
In particular, the instant when the value of the control voltage VZCD goes to zero during resonance, which corresponds to the instant when the phase voltage Vph on the first internal node N1 is equal to the input voltage Vin, is determined.
Even though the solution described makes it possible to obtain as a whole a good control performance, the present Applicant has found that it also has some drawbacks.
In particular, as indicated previously, controlling switching of the switch element 6 requires detection of the output voltage Vout by the resistive divider 10b, which involves, however, a considerable power consumption.
The resistance of the resistors used in the resistive divider is indeed high in order to minimize current leakages; for example, it is of the order of tens of MΩ. Considering a value of 400 V for the output voltage Vout, the resistors thus entail a power consumption of approximately 16 mW. Considering further that the entire power converter may have a target power consumption not higher than 60 mW, the consumption associated to the resistive divider 10b amounts to 25% of the total power consumption.
Furthermore, it is clear that a specific pin 2b is required in the control device 2 for reading the value of the output voltage Vout, with a consequent increase of the dimensions of the package and of the manufacturing costs.